I've posted about this before and today I was looking into this phenomenon again. I happened to stumbled across my own old post and was reading it, thinking "this sounds exactly like my situation, and the guy even lives is Tennessee! What are the odds!" then I realized it was my own old post that I had forgotten about... Anyways new thread lol

I was listening to the HPA podcast with Paul Yaw from Injector Dynamics today at work and it significantly improved my understanding of how fuel injectors work, peak and hold injectors, etc., but it reminded me of this issue. A lot of people on here always point at the IAT sensor for the hot restart issues, or fuel vaporizing due to heat, or whatever else they can come up with. But today I did a little more research and I've got a good hypothesis as to what causes the heat related fueling issues. So hear me out and feel free to critique or add something in.

The issue: Hot restarts, Lean cruise and Idle with warmer weather/Rich cruise and idle in cold weather.

Info about my car: Stock return-less fuel system ('99 NB) with a Walbro 255lph pump and Flowforce 960cc injectors (formerly FF 640cc injectors).

The hypothesis: High injector temperatures cause the deadtime to increase. The heat can either come from variations in weather or even worse, the hot engine's head while at idle or not running.

Why do I think this?:
I began on this thread (HPA Forums - Dont know if linking to other forums is allowed, can remove if not), in which the OP was explaining a fueling variation of 10-15% or more. He has verified Injector deadtimes, MAT correction, Coolant temp, fuel pressure, and his MAP is stable. He is also running 1150cc Injectors. Same issue as us miata folk are reporting. To which a VIP member by the name of "Slides" chimed in saying that injector temperature sensitivity is a known issue. He also states that some 1600cc injector regularly gives people issues. (Take note of how this is common on larger injectors, could just be due some sort of sample bias on my part, but it'll feed into my hypothesis later. Another user says he has the same issue up to 20% change most notably at low puslewidth... see where I'm going with this?
From here I started to do some googling. I found some papers on diesel direct injectors, but not much on gasoline injectors. Anyways, here is what I found:

• This SAE paper regarding diesel common rail injectors says in the summary that the results showed a "strong effect of thermal conditions on the injector hydraulics. The injected volume can be varies up to 30% compared to the reference operating condition (Tfuel = 40 deg C, Tbody = 90 deg C) The injection rate analysis evidenced that the injector closure timing can be seriously affected by the system thermal state, while the nozzle steady flow is typically less influenced by the fuel and injector body temperature in the examined range". So what they're saying is the injector closing time is a lot faster when it is hot resulting in less fuel.
• This journal's abstract says that "with the increase of fuel temperature, the injection quantity and maximum injection rate decreased, the time of start injection was delayed, and the peak fuel pressure value decreased." This is very similar to the diesel injectors mentions before, except here the start of injection was delayed. This can be interpreted as the deadtime of the injector being increased when hot, and decreasing when cold (to me at least).
• This patent's description says that the "use of the high resistance coil require that the holding current, for holding the injector open, generates significiant heat in the coil. The generated heat raises the temperature of the injector coil, thereby changing its resistance and hence its operating time and the fuel flow characteristics of the injector." which again sounds like heat is effecting the deadtime of the injector, so I did a quick google search regarding how temperatures effect DC solenoids (which seem to me to be similar in theory).
• That google search got me to this page which states that "As a solenoid heats up, its pull force decreases. The coil resistance increases and flowing current decreases as the temperature rise of the solenoid and its ambient temperature rise when energized, resulting in the magnetomotive force to be smaller." which again makes it sound like a hotter injector has less opening force leading to increased deadtimes.

So, yeah. It really sounds like all of our hot restart issues, lean conditions on hot days, or rich conditions on cold days can be explained by my current hypothesis. Which brings me to the question "Why have I not seen or heard of any sort of aftermarket solution to correct the injector deadtimes using fuel temperature?". Fuel temperature sensors exist, but they're often explained as being used to compensate for the .1% density change per degree C. This isn't a big enough percentage to cause our issues though. Could this just be a misunderstood use of the sensor, and that OEMs are actually correlating this to injector temperatures? I do think using this as a basis for compensation could be beneficial, however it wouldn't account for the heat generated by the injector ports being on the side of the engine's head. From a quick google I'm seeing that a cylinder head can reach as high as 200 deg C, which would definitely exacerbate this issue since the injectors are sealed to the head.

Last thing, I often see people speculating about how Mazda dealt with hot restarts and the lean condition that we are experiencing. I have an explanation for that too: They didn't need to. The stock injectors' minimum pulse width is likely much higher due to their size, so an increase of, say, .2ms of deadtime is negligible. 0.2ms of a 3ms pulse is 6% on small injectors but a .2ms deadtime increase on a 1.7ms pulse is 11% with bigger injectors (not actual number, just something I've pulled out of my ***). It just happens that this is also about how far off our fueling is when experiencing this phenomenon.

So, thoughts, comments, or questions? Anyone with actual programming experience want to look into this further? I'm thinking about trying to reach out to Paul Yaw from Injector Dynamics to see if he has any input on my hypothesis. Also considering crossposting to MSExtra forums in hopes of reaching somebody in development of the MS firmware, but I realize this is not very likely to be included in a future firmware release.

 

Compelling hypothesis, Professor Watterson! I'm not in any way qualified to opine on the matter at hand, but I'm going to anyway.

Hot restarts drive me mad and I never figured out how to fix them. FF 640cc here.

If the problem is caused by injectors getting heat soaked then a 3D IAT fuel trim table with IAT vs engine runtime on the axes would be a good workaround as outlined in the link.

 

TLDR, but I encourage the thinking. Heat soaked injectors changing dead time is kinda common knowledge. The solution to hot restarts on MS is as simple as adding ASE back as CLT increases. The extra fuel compensates for the added dead time until the injectors cool.

use time based (0.1 second) cycles



Lean idle and cruise are fixed by a properly configured EGO.

 

Sounds like an opportunity for an aftermarket solution. Heat sinks for large injectors. Hell, why not water cooling?

Hey, if 100MPG carburetors were a thing, why not this?

 

Once you get fuel that hasn't been sitting in the rail flowing through them they cool really quickly. +1 for return style. The ambient temps around the rail while "heat soaking" don't allow for a convenient passive solution to remove the heat. IIRC you can setup MS to run the fan when the car is turned off, which would cool the bay dramatically, but then you have to run a separate circuit that isn't tied to the ignition switch or main relay.

 

See both of these kinda are just bandaid solutions, and I’ve employed a similar table to what you’re showing Ted75zcar. I just hate having to put a bandaid over something that need stitches if that makes sense. Also, way to pop my bubble haha. I’ve never heard of any of this “common knowledge”.

With the huge temperature swings of as much as 20-100° F throughout the year where I’m at, I end up needing as much as 15% ego correction at cruise in either direction. I don’t feel comfortable giving my wideband sensor that much control. I guess I’ll tune the car to be rich and work up really aggressive PID settings. Time to do some reading on that I guess.

 

Is there any correlation between the Flowforce injectiors and the severity of this issue. I used the method that Ted describe in post 3# to get past it, but have wondered if my FF640's were part of the cause or adding insult to injury.

 

I wouldn't get too bent out of shape about the "common knowledge" thing. When I say that, I mean for the people who really understand this stuff, this is well known. Reality is only a small percentage of those who dabble actually understand what they are doing and why. I mentor many JR engineers, and always push them to discover the "answer" organically, as in the long run this leads to a better fundamental understanding, as opposed to just doing what somebody else did because it worked for them.

If you think about this particular problem you will probably discover that the solution doesn't relate to heat removal or air or fuel temperature. I will leave it to you to discover why.

The best way to determine the state of the actuator is to use the current and voltage waveforms. There is a strong correlation.

In reality, ASE is actually a **** that does exactly what you are looking for, using the most representative real-time data that is available and directly controlling the process you want to effect. It even has a delay timer to cancel the correction for when the injectors match the fuel temp and not the block temp. That giant piece of hot metal has reached a steady state temperature, and those tiny pieces of metal attached are simply along for the ride. Once fuel is flowing through them, they hit the average fuel temp pretty fast, regardless of the underhood temps.

Different actuators (injectors) will have different characteristics. It is very plausable that a FF injector will behave differently than say an ID.

As far as EGO is concerned, with a good fuel table the EGO integrator term will servo to an average correction based on numerous things that we can and can't control. Giving a well setup EGO lots of authority is really not a big deal, because you can easily setup multiple orthogonal safeguards to catch failure modes. Now 15% swings are pretty extreme, especially if you are not seeing a constant average under relatively steady state conditions, like cruise. The FW in the 1.5 ish time frame saw a hugely beneficial update where EGO isn't reset during transient events. I highly recommend getting this version if you don't already have it.

FWIW, under 100kpa I give EGO all of it wrt authority. Above 100 I decrease the negative authority quite a bit, but still allow as much positive as it wants. Safeties for AFR, fuel pressure, and fault conditions in the wideband are all in place and active. you shouldn't have to get aggressive with the time constant to make EGO your best friend. If the process output is swinging wildly, your VE table and AE probably need attention. Get that delay table setup too.

 

I was wondering the same. Maybe someone running ID injectors or with a lot of tuning experience could chime in?

Oh, no I wasn't getting bent out of shape. I was just joking about the common knowledge part. It's hard to convey sarcasm through text.
Also, I'm still on an MS2PnP so I don't think a lot of what was mentioned is available to me. I don't think it's the VE table or AE, both do a good job of reaching AFR targets and being smooth (assuming it is the temperature at which I tuned the car). I'm tempted to buy the MS3 that's for sale right now, but I really shouldn't at the moment. I've still got a lot of stuff that is more important regarding the car, plus I'm wanting to move this summer so I can't just be throwing money around anymore. Maybe I'll build one over summer to save money and learn more.

Last thing... by "If you think about this particular problem you will probably discover that the solution doesn't relate to heat removal or air or fuel temperature. I will leave it to you to discover why.
The best way to determine the state of the actuator is to use the current and voltage waveforms. There is a strong correlation.", are you suggesting that the reason of the AFR swings would be presented in the waveforms themselves, or am I misinterpreting this?

Unfortunately, I'm not aware of any way to hook up an oscilloscope in the car, so I'll have to take your word for it if so haha. I'm guessing what you mean by this is that when the injector is hot, there would be more resistance and therefore less peak current which causes less opening force? Something like what is shown here, just a lower peak?

Also, how would that not relate to temperature of the temperature affects the resistance? (Not trying to argue, just trying to better understand the subject. Want to make sure it doesn't come across that way.)

 

Unfortunately, you are going way down the rabbit hole if you are trying to anything at this level of detail with MS2.

That link does have hints as to the voltage/current waveform artifacts that indicate the opening or closing of the injector. The dead time is the duration of time from trigger to the open artifact minus the amount of time from release to the closed artifact. If you look through my build thread (starting at #81) I posted the waveforms with descriptions for a solenoid valve like what is used for EBC. The characteristics are similar be it a relay, solenoid valve, injector.... these are aspects of using magnetic fields to move things around.

Edit: some of the things you have said lead me to believe you don't have incorporate AFR turned on.

 

I have the exact same issue described in this thread. I'm also on FF640. Was going to tinker with this and try what is recommended with ASE % and Taper as it's starting to heat up around here it's a good time to try to get something working.

Thanks for the info-
MM

 

Yep. I do think you are on to something there.

Also fuel density may decrease with heat as well.

I run 1500cc and e85 so my experience has been similar and in my area we have days with a heat index of over 110f on some hot summer days. I have yet to do a post on my thread what I've done as I've mostly been working on torque converter adapters lately. However I did recently install a flex fuel sensor to allow me to measure fuel temperature and adjust fueling based off fuel temperature. That does help. I can tell you the fuel gets really hot. I run a walbro 525 pump which draws a lot of current and adds a fair amount of heat to the fuel. Since I run a return setup it just continually heats whats in the tank and gets really bad as tank volume goes down.

I've tossed around the idea of doing a fuel cooler too. I know back in my early days many people ran cool cans with ice in them with carburetors for drag racing. I never tried that but seeing how the fuel gets heated and how adjusting fuel based off that heat increase leads me to believe there may be a performance increase if I can lower that fuel temp which may in turn lower the overall temperature of whats going into the engine (both fuel and air) since I'm spraying a pretty large quantity of ethanol at full load.